How to determine proper bed depth

Hi,
I have a 10x54 iron filter filled with birm. It has an old fleck backwashing valve which is no longer functioning. I removed the control head to examine the media and was surprised to find the birm packed tightly into the vessel all the way to the threads.

How do I determine the appropriate bed depth and clear area between the bed and control head for this tank?

My plan is to have a complete water test done and then to purchase an appropriate filtering system. In the meantime I will remove the media, wash and repack the tank.

The water quality is actually quite good after the birm filter. We have no staining or residue on the fixtures. All of the nitrate/nitrites/iron bacteria tests came back negligible from the town health dept. They do not test for hardness, iron, etc....

The only nuisance is the bottom of the tea pot gets coated in a black powdery residue after a few weeks of use. No other cooking utensils develop this residue from boiling water. I will have the residue tested as well.

17" of what is called head space.
It is the distance from the top of the tank to the media on the inside, there is some debate as does one start at the top or start just under the threads and then start counting the inches.